1. Field of the Invention
The invention relates to a III-V system compound semiconductor device and a method for manufacturing the semiconductor device.
In recent years, it is required to realize a large-capacity and high-quality optical telecommunications system or a high-performance and high-speed computer, and in order to meet such requirements it is necessary to greatly improve semiconductor devices composing their respective electronic circuits in performance.
A heterojunction bipolar transistor (HBT) in particular attracts public attention as one of promising semiconductor devices and is expected to greatly improve semiconductor devices in speed.
In order to greatly improve such an HBT in speed, it is indispensable to reduce its base resistance by forming a high-density p-type base layer in a crystal growth process of the HBT.
2. Description of the Prior Art
Up to now, beryllium (Be) has been used as a p-type dopant for forming a p-type base layer of an HBT using a GaAs or InGaAs system compound.
Recently, however, carbon (C), which is smaller in diffusion coefficient than Be and therefore has less variation in distribution of impurities caused by a heat treatment after introduction into a semiconductor, has attracted attention.
And CCl.sub.4 is reported as one of promising C dopant materials for GaAs or InGaAs crystals (reference literature: T. P. Chin et al. Appl. Phys. Lett. 59(22), November 1991).
However, since use of CCl.sub.4 is scheduled to be completely prohibited by 1995 according to Montreal Protocol in relation to materials destroying ozone layers, it is an urgent necessity to develop a substitute material for CCl.sub.4 as a C dopant material for giving a p-type conductivity to a GaAs crystal layer or particularly to an InGaAs crystal layer.
Up to now, an example in which C has been doped into a GaAs crystal by means of an MOCVD method as using CH.sub.2 I.sub.2 instead of CCl.sub.4 as a C dopant material has been reported (reference literature; T. F. Keuch et al. Mat. Res. Soc. Symp. proc. Vol. 204. p171, 1991).
According to these researches, however, density of positive holes in a GaAs crystal layer cannot be made higher than 7.times.10.sup.17 /cm.sup.3 in case of growing the GaAs crystal layer by means of an ordinary MOCVD method as using CH.sub.2 I.sub.2 as a C dopant material, and such density of positive holes as this degree fails in forming a high-density p-type base layer indispensable to a high-performance HBT.
Therefore, it is necessary to develop a crystal growing technology for growing such a binary system crystal layer as GaAs or such a ternary system crystal layer as InGaAs whose maximum density of positive holes is 10.sup.18 /cm.sup.3 or higher as using CH.sub.2 I.sub.2 as a C dopant material and to determine its crystal growth condition.
There has not been an example of growing a ternary system p-type InGaAs crystal layer and the like as using CH.sub.2 I.sub.2 as a C dopant material.